The Hero Masks Project

Here is a chart showing the most common mask types:

Source: Comparison of Mask Standards, Ratings, and Filtration Effectiveness by Smart Air Filters

Data Source: Testing the Efficacy of Homemade Masks: Would They Protect in an Influenza Pandemic?
Chart Source: HOMEMADE MASKS IN A TIME OF SHORTAGE

• Effectiveness of face masks used to protect Beijing residents against particulate air pollution (ncbi.nlm.nci.gov)
  • Consumer masks in Bejing had problems with fit affecting effectiveness
• Respiratory Protection for Exposures to Particulate Air Pollution (multimedia.3m.com)
  • Summary of mask effectiveness and part numbers/styles
• “The Cambridge Study”:  PDF version
  • Summary of results in easier to read chart form :
    
  • Testing the Efficacy of Homemade Masks: Would They Protect in an Influenza Pandemic?
    • Conclusion: A protective mask may reduce the likelihood of infection, but it will not eliminate the risk, particularly when a disease has more than 1 route of transmission. Thus any mask, no matter how efficient at filtration or how good the seal, will have minimal effect if it is not used in conjunction with other preventative measures, such as isolation of infected cases, immunization, good respiratory etiquette, and regular hand hygiene. An improvised face mask should be viewed as the last possible alternative if a supply of commercial face masks is not available, irrespective of the disease against which it may be required for protection. Improvised homemade face masks may be used to help protect those who could potentially, for example, be at occupational risk from close or frequent contact with symptomatic patients. However, these masks would provide the wearers little protection from microorganisms from others persons who are infected with respiratory diseases. As a result, we would not recommend the use of homemade face masks as a method of reducing transmission of infection from aerosols.
    • Testing the Efficacy of Homemade Masks: Would They Protect in an Influenza Pandemic? (ncbi.nlm.ih.gov, 2013-05-02)
      • Result: The median-fit factor of the homemade masks was one-half that of the surgical masks. Both masks significantly reduced the number of microorganisms expelled by volunteers, although the surgical mask was 3 times more effective in blocking transmission than the homemade mask.
      • Conclusion: Our findings suggest that a homemade mask should only be considered as a last resort to prevent droplet transmission from infected individuals, but it would be better than no protection. (Disaster Med Public Health Preparedness. 2013;0:1–6)
  • Face masks to prevent transmission of influenza virus: a systematic review (cambridge.org, 2010-01-22)
    • Summary: … There is some evidence to support the wearing of masks or respirators during illness to protect others, and public health emphasis on mask wearing during illness may help to reduce influenza virus transmission. There are fewer data to support the use of masks or respirators to prevent becoming infected. Further studies in controlled settings and studies of natural infections in healthcare and community settings are required to better define the effectiveness of face masks and respirators in preventing influenza virus transmission.

• Alternative Materials to use
  • Vacuum cleaner bags (claims of them containing fiberglass debunked here and here) 
• Homemade Masks In a Time of Shortage (Hackaday.com)
  • References some research on effective materials, and a comment section with lots of good information